1. Field of the Invention
The present invention relates to improved wettable polymeric hydrogel compositions useful for the production of biomedical devices, especially contact lenses.
2. Background
Hydrogels have been a desirable class of material for the preparation of biomedical devices, and have been known since at least Wichterle, et al U.S. Pat. No. 3,220,960 which disclosed hydrogels comprising a hydrated polymer of a hydroxyalkyl acrylate or methacrylate crosslinked with a corresponding diester (poly 2-hydroxyethyl methacrylate, known as poly-HEMA).
A hydrogel is a hydrated crosslinked polymeric system that contains water in an equilibrium state. The physical properties of hydrogels can vary widely and are mostly determined by their water content. Since hydrogels exhibit excellent biocompatibility, there has been extensive interest in the use of hydrogels for biomedical devices, especially contact lenses.
In the field of contact lenses, various factors must combine to yield a material that has appropriate characteristics. Oxygen permeability, wettability, material strength and stability are but a few of the factors which must be carefully balanced to achieve a useable end-result contact lens. Since the cornea receives its oxygen supply exclusively from contact with the atmosphere, good oxygen permeability is a critical characteristic for any contact lens material.
It was discovered that certain crosslinked polymeric materials could be hydrated and retain their water content. It was further found that the higher the water content within contact lenses made from these crosslinked hydrogel polymers, the greater was the oxygen permeability through the lens to the cornea.
High water-containing hydrogels have at times exhibited undesirable mechanical properties. For example, such hydrogels are often not easily formed into hydrolytically stable lenses. Further such materials have at times exhibited tearing or other breakage as a result of poor tensile strength. What was needed was a highly oxygen permeable material that was durable and highly wettable. Wettability is important in that, if the lens is not sufficiently wettable, it does not remain lubricated and therefore cannot be worn comfortably in the eye. The optimum contact lens would have not only excellent oxygen permeability, but also excellent tear fluid wettability.
Silicone-containing materials were tried as viable contact lens materials and displayed very good oxygen permeability and durability. However, most silicone-containing materials are largely hydrophobic and therefore not sufficiently wettable. Further, it is believed that such hydrophobicity causes enhanced deposit problems, which may also result in discomfort when wearing contact lenses made from certain silicone-containing polymers.
Therefore, an optimal hydrogel material for biomedical devices, such as contact lenses, would have ideal rigidity, high oxygen permeability and a high degree of wettability.